1. Field of the Invention
The present invention relates to the transfer of workpieces such as semiconductor wafers from a storage and transport pod to a process tool, and in particular to a system for ensuring a firm engagement between, and removal of particulates, contaminants and cleanroom air from, the port and pod doors as the pod door is removed from the pod and stowed in the process tool during workpiece transfer between the pod and process tool.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironment without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled"SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (.mu.m) to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half .mu.m and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles and molecular contaminants become of interest.
SMIF pods are in general comprised of a pod door which mates with a pod shell to provide a sealed environment in which wafers may be stored and transferred. So called"bottom opening" pods are known, where the pod door is horizontally provided at the bottom of the pod, and the wafers are supported in a cassette which is in turn supported on the pod door. It is also known to provide "front opening" pods, in which the pod door is located in a vertical plane, and the wafers are supported either in a cassette mounted within the pod shell, or to shelves mounted in the pod shell. For both front opening and bottom opening pods, a pod door includes an interior surface which is included as part of the sealed pod environment, and an exterior surface which is exposed to the environment of the wafer fab.
In order to transfer wafers between a SMIF pod and a process tool within a wafer fab, a pod is typically loaded either manually or automatedly onto a load port on a front of the tool. The process tool includes an access port which, in the absence of a pod, is covered by a port door which includes an exterior surface exposed to the wafer fab environment and an interior surface which is part of the sealed environment within the process tool. The SMIF pod is seated on the load port so that the pod door and port door lie adjacent to each other. Registration pins are provided on the port door that mate with holes in the pod door to assure a proper alignment of the pod door with respect to the port door.
Once the pod is positioned on the load port, mechanisms within the port door unlatch the pod door from the pod shell and move the pod door and port door together into the process tool where the doors are then stowed away from the wafer transfer path. The pod shell remains in proximity to the interface port so as to maintain a clean environment including the interior of the process tool and the pod shell around the wafers. A wafer handling robot within the process tool may thereafter access particular wafers supported in the pod for transfer between the pod and the process tool.
It is extremely important to provide a clean environment around the exposed wafers within the process tool. While the air within wafer fabs is typically filtered to some degree, the environment surrounding the process tools and SMIF pods includes relatively high levels of particulates and contaminants as compared to within the pods and tools. As such, significant steps are taken to isolate SMIF pod and process tool interiors from the surrounding environment within the fab.
As explained above, the pod door and port door, even though having surfaces exposed to the environment of the wafer fab, are typically brought into the interior of the process tool in preparation for wafer transfer between the pod and the tool. In order to prevent the particulates and contaminants on the exposed door surfaces from contaminating the interior of the process tool, it is known to hold the exposed pod and port door surfaces against each other when bringing the pod and port doors into the process tools and while the doors are positioned therein. For example, U.S. Pat. No. 4,534,389, entitled "Interlocking Door Latch For Dockable Interface For Integrated Circuit Processing", discloses a spring loaded latch and release cable (FIG. 5 of that Patent) for holding a pod door against the port. Such contact may trap particulates and/or contaminants between the exposed surfaces to thereby prevent the transfer of the particulates and/or contaminants into the process tool.
Coupling mechanisms are known for coupling the pod door to the port door as the pod door is removed from the pod and stowed in the process tool. In addition to these coupling mechanisms, the registration pins fit within registration holes in the pod door to guide the pod door into a proper position on the port door. However, a tolerance is provided between the registration pin and hole to facilitate seating of the pin within the hole without excessive wear. As a result, without additional restraints between the pod and port doors, it is possible that the pod door will vibrate, tilt or move around on the port door. Any such vibration or movement may result in particulates and/or contaminants dislodging from the pod and/or port door surfaces and settling in the process tool.
Prior art attempts have been made to hold the pod door firmly against the port door while the doors are coupled together and stowed in the process tool. Such systems are disclosed for example in U.S. Pat. No. 5,291,923, entitled "Door Opening System and Method", which patent is assigned to IBM, and in U.S. Pat. No. 5,772,386, entitled"Loading and Unloading Station for Semiconductor Processing Installations", which patent is assigned to Jenoptik A. G. ("the '386 Patent"). As set forth in the '386 Patent for example, the port door includes a pair of suction cups connected to a vacuum source. When the pod door is coupled to the port door, the suction cups engage a portion of the surface of the pod door, and a vacuum source creates suction within the cups to hold the pod door. The cups retract into bore holes to hold the surfaces of the pod door and port door together. The vacuum cups of the '386 Patent engage a small area on the pod door for the purpose of gripping the pod door. The vacuum system of the '386 Patent is unconcerned with removing particulates or contaminants from the pod and port door surfaces.
In conventional systems, as the pod door is positioned adjacent the port door for coupling therewith, it is possible that the pod door may be misaligned with respect to the port door. This misalignment may be caused by one of several contributing factors. In front opening systems, the pod is seated on a support surface on the load port, and the engagement between the pod and the support surface is relied upon to align the pod door to the port door. However, if the pod is warped or manufactured to poor tolerances, the pod door may not precisely align with the port door. Another potential cause of misalignment between the pod and port doors may be that an upstream load port has returned the pod door to the pod in an off-center position with respect to the pod shell.
The port door typically includes registration pins which are intended to align with and fit within registration holes on the juxtaposed surface of the pod door. These registration pins may have spherical tips and/or may be flexibly mounted to the port door (such as for example as disclosed in the '386 Patent) so that as a misaligned pod door is brought to the port door, the registration pins adjust the position of the pod door to be correctly aligned with respect to the port door. Where the misalignment is caused by an upstream load port, it is desirable that the registration pins adjust and correct the position of the pod door so that it may be returned to a centered position with respect to the pod shell. However, where the pod is warped or built to poor tolerances, a pod door may be centered within the pod shell, but it may be misaligned with respect to the port door. In this instance, adjustment of the position of the pod door by the registration pins may result in the pod door being returned to the pod shell in an off-center position, and as a result, the sides of the pod door may scrape against the pod shell. Any such scraping may generate harmful particulates. In prior art systems including port doors that adjust the position of the pod door, the position of the door is adjusted to an expected center position by the registration pins regardless of the cause of the misalignment.
In conventional wafer-carrying pods, as explained above, the pod door typically includes a latch plate assembly and cast walls which define wells and slots for receiving the registration pins and latch keys. Other than these mechanisms, the pod door includes a predominantly hollow interior volume. The outer walls of the pod door typically include one or more openings to the external environment, such as for example at the slots through which the latch plates extend to couple the pod door to the pod shell. Thus, when a pod is first introduced onto the load port of a process tool, the interior volume of the pod door typically includes air from the cleanroom surrounding the minienvironment.
When the pod door is brought into the minienvironment, the cleanroom air from within the interior of the pod door may introduce particulates and contaminants to the minienvironment as the air from within the pod interior diffuses into the minienvironment. This may be particularly harmful where the interior of the minienvironment includes an inert gas or gasses other than air.